Carbon and graphite bodies are porous and many products fabricated from carbon or graphite require the carbon or graphite stock to be impregnated with a suitable impregnant such as a polymer resin or pitch to decrease its porosity and increase strength. Resins are subject to certain inherent disadvantages in that in general, they have high viscosities, low carbon yields and tend to form a glassy non-graphitizing carbon. For carbonaceous electrodes it is more common to use a pitch as the impregnant. Pitch is a complex mixture of polynuclear aromatics generally derived from the thermal treatment of coal tar or a petroleum tar. Typical impregnating pitches have coking values (MCC) of about 40-50%. However, these typical impregnating pitches are solid at room temperature and must be preheated to high temperature to transform them to a low viscosity liquid suitable for impregnation. It is also conventional to preheat the carbon or graphite electrode stock to an elevated temperature before adding the pitch impregnant. The electrode is then cooled to solidify the impregnant.
Pitch admixed with a polymerizable liquid has also been used for impregnation. A typical impregnant admixture for impregnating carbonaceous electrodes in the manufacture of chlorine includes a liquid solution of furfuraldehyde, a No. 15 vacuum coal tar pitch and a diethylsulfate catalyst. This is taught in U.S. Pat. No. 3,375,132 the disclosure of which is herein incorporated by reference.
After pitch is impregnated into the carbon or graphite body it is normally rebaked to carbonize the impregnant. About 1/3 of a conventional pitch impregnant composition runs out of the carbon body prior to coking thereby decreasing the ultimate impregnant carbon yield after carbonization to about 35%. The pitch which runs out of the rebaked stock presents a disposal problem and may also foul up the baking furnace. Currently a method employed to minimize the run out of pitch from the impregnated stock is to bake the electrode very slowly. This decreases productivity and increases production cost.